Andrei Vagin <avagin@virtuozzo.com> writes:
> On Tue, Oct 25, 2016 at 04:45:44PM -0500, Eric W. Biederman wrote:>> That is certainly interesting. The problem is that the reason we were>> going slow is that there were in fact mounts that had not been traversed>> in the share group.>> You are right.>>> >> And in fact the entire idea of visiting a vfsmount mountpoint pair>> exactly once is wrong in the face of shadow mounts. For a vfsmount>> mountpoint pair that has shadow mounts the number of shadow mounts needs>> to be descreased by one each time the propgation tree is traversed>> during unmount. Which means that as far as I can see we have to kill>> shadow mounts to correctly optimize this code. Once shadow mounts are>> gone I don't know of a case where need your optimization.>> Without shadow mounts, it will be hard to save predictable behaviour> for cases like this:>> $ unshare --propagation private -m sh test.sh> + mount -t tmpfs --make-shared zzzz A> + mkdir A/a> + mount -t tmpfs zzzz A/a> + mount --bind A B> + mount -t tmpfs zzzz B/a> + grep zzzz> + cat /proc/self/mountinfo> 155 123 0:44 / /root/tmp/A rw,relatime shared:70 - tmpfs zzzz rw> 156 155 0:45 / /root/tmp/A/a rw,relatime shared:71 - tmpfs zzzz rw> 157 123 0:44 / /root/tmp/B rw,relatime shared:70 - tmpfs zzzz rw> 158 157 0:46 / /root/tmp/B/a rw,relatime shared:72 - tmpfs zzzz rw> 159 155 0:46 / /root/tmp/A/a rw,relatime shared:72 - tmpfs zzzz rw> + umount B/a> + grep zzzz> + cat /proc/self/mountinfo> 155 123 0:44 / /root/tmp/A rw,relatime shared:70 - tmpfs zzzz rw> 156 155 0:45 / /root/tmp/A/a rw,relatime shared:71 - tmpfs zzzz rw> 157 123 0:44 / /root/tmp/B rw,relatime shared:70 - tmpfs zzzz rw>> X + a - a = X>> Maybe we need to add another ID for propagated mounts and when we> do umount, we will detach only mounts with the same propagation id.>> I support the idea to kill shadow mounts. I guess it will help us to> simplify algorithm of dumping and restoring a mount tree in CRIU.>> Currently it is a big pain for us.
Killing shadow mounts is not exactly a done deal as there are some user
visible effects. The practical question becomes do we break anything
anyone cares about in userspace. Answering those practical questions
sucks.
I definitely think we should try to kill shadow mounts because they are
such a big pain to deal with, and only provide very limited value.
So far the only thing I have seem shadow mounts being good for is
preserving unmount behavior in cases where what someone has
constructed an artificially evil mount tree. I haven't figured out how
to see how any of those mount trees are actually useful in real life.
Eric